1. Field of the Invention
The present invention relates to a wireless communication device and a communication terminal that perform a communication by use of sound packets.
2. Description of the Related Art
When a user carries a wireless voice communication system in one's hand, such as a cellular phone and a cordless phone, or uses the system at a location where ambient circumstances change, variations occur in received signal strength indicator, a distance between devices, and an interference radio wave, which greatly affects communication quality. A known countermeasure against the problems is; for instance, a technique described in relation to a wireless telephone system as disclosed in JP-T-2002-509387.
The wireless telephone system of JP-T-2002-509387 is made up of a base unit that has a base transceiver and at least one wireless handset and that is connectable with one external phone line. The wireless handset and the base unit implement communication by means of fixed-size audio packets, each of which is composed of a plurality of audio data samples and error correction bits. Moreover, JP-T-2002-509387 includes descriptions about changing a relative number of bits assigned to an audio data sample and an error correction bit in connection with a packet configuration of a subsequent audio packet by means of monitoring the quality of a digital link set on a wireless channel and determining whether or not a change has occurred in error rate.
Specifically, when the error rate becomes worse, the wireless telephone system performs processing for decreasing an amount of data pertinent to an audio data sample contained in a packet and increasing an amount of data pertinent to error correction bits to thus assign a larger number of bits to error correction, thereby broadening a distance range and enhancing a tolerance to the interference radio wave.
JP-A-2010-154163 discloses a technique of reversing least significant bits in “n” bits in accordance with the number of “1s” in the “n” bits of n-bit ADPCM data, thereby letting the least significant bits act like a parity signal and enabling detection of an error without involvement of an increase in the number of bits. As a result, when a communication environment becomes worse, sound quality is somewhat distorted, but accuracy in error detection is increased. When an error occurs in ADPCM data, the ADPCM data themselves can be switched to a muted or attenuated state.
In addition, JP-A-7-143074 discloses a technique of converting an ADPCM code for which an error is detected into an ADPCM code which may cause a much smaller amplitude change in a decoded signal, thereby preventing occurrence of high-level unusual noise and avoiding a situation in which a level of a normal decoded sound will fall more than necessary to thus prevent a listener from feeling unpleasant sensation.